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Recovery Rate of Frac Fluids and Provenance of Produced Water from Unconventional Shale Exploration in Saudi Arabia

Abstract

Hydraulic fracturing of unconventional shale to stimulate wells and recover natural gas typically implies the injection of water with sand and chemicals at high pressures into the wellbore to create small (< 1.0 mm) fractures. As a standard technique, supply water from nearby surface or close-to-surface resources is mixed with a variety of additives, such as diluted acid, gelling agent, and KCl to optimize the fracking process. During the post-frac stage, a variety of chemically distinct water types are produced from fracturing wells, resulting in technical challenge to characterize the type and origin of produced water. The present paper deals with a specific case study on hydraulic fracturing of an exploration well in Saudi Arabia. A total of 41 flowback water samples from the fracture-stimulated Qusaiba Hot Shale (QHS) were collected and analyzed for major ions and trace elements (B, Ba, Br, Fe, Li, Sr), environmental (δ2H, δ13C, δ18OH2O, δ18OSO4, δ34SSO4, δ37Cl, 87Sr/86Sr) and cosmogenic isotopes (3H, 14C, 36Cl) to trace type and provenance of flowback water during the post-frac period, as well as to quantify recovery rates for frac fluids. After the initial stage of the post-frac period, flowback of slickwater and KCl mud filtrate were replaced by Na-Cl type formation water, with an average salinity of 50,000 mg/L. Less than 10% of the total injected fluids were recovered during the post-frac period, while 78.8% of the total flowback is composed of formation water (20,843 bbls out of 26,446 bbls). Radiogenic age dating proved the infiltration of meteoric surface water (and partially evaporated seawater) into the present Qusaiba Hot Shale interval during the Early Holocene Pluvial Period (14C age: 7,900 yr BP), indicating the presence of a very recent, dynamic hydraulic flow system. Low 36Cl/Cl ratios for QHS formation water (31 - 102 × 10E-15) indicate the location of the recharge zone close to coastal areas. The thermodynamic equilibrium between calculated water temperatures and measured reservoir temperatures suggests an in situ provenance of the produced water from the Qusaiba Hot Shale interval. Feeding of petrophysical & structural reservoir models with data on groundwater dynamics can provide, a) evidence for the functionality of natural or hydraulic fractures on gas mobilization, b) potential pathways for gas migration, and c) quantitative tools to evaluate the benefit and efficiency of hydraulic fracking.